[AIRFLOW-1614] Replace inspect.stack() with sys._getframe()
[AIRFLOW-1614] Replace inspect.stack() with sys._getframe()

inspect.stack() is really expensive, and slows
down processing of dags
having large numbers (100s, 1000s) of subdags.

Closes #2610 from gbenison/gcbenison2

lxneng/incubator-airflow,nathanielvarona/airflow,fenglu-g/incubator-airflow,wooga/airflow,Acehaidrey/incubator-airflow,gilt/incubator-airflow,gtoonstra/airflow,wndhydrnt/airflow,cfei18/incubator-airflow,jfantom/incubator-airflow,gilt/incubator-airflow,gtoonstra/airflow,edgarRd/incubator-airflow,akosel/incubator-airflow,apache/airflow,edgarRd/incubator-airflow,edgarRd/incubator-airflow,owlabs/incubator-airflow,gilt/incubator-airflow,edgarRd/incubator-airflow,r39132/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,r39132/airflow,adamhaney/airflow,sergiohgz/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,DinoCow/airflow,lyft/incubator-airflow,KL-WLCR/incubator-airflow,fenglu-g/incubator-airflow,yk5/incubator-airflow,cjqian/incubator-airflow,mrkm4ntr/incubator-airflow,jhsenjaliya/incubator-airflow,ProstoMaxim/incubator-airflow,apache/incubator-airflow,jhsenjaliya/incubator-airflow,janczak10/incubator-airflow,subodhchhabra/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,yk5/incubator-airflow,sekikn/incubator-airflow,hgrif/incubator-airflow,akosel/incubator-airflow,skudriashev/incubator-airflow,wndhydrnt/airflow,sergiohgz/incubator-airflow,mistercrunch/airflow,hgrif/incubator-airflow,CloverHealth/airflow,wolfier/incubator-airflow,RealImpactAnalytics/airflow,RealImpactAnalytics/airflow,Tagar/incubator-airflow,andyxhadji/incubator-airflow,zack3241/incubator-airflow,artwr/airflow,dhuang/incubator-airflow,KL-WLCR/incubator-airflow,skudriashev/incubator-airflow,airbnb/airflow,wolfier/incubator-airflow,cfei18/incubator-airflow,jhsenjaliya/incubator-airflow,KL-WLCR/incubator-airflow,sekikn/incubator-airflow,OpringaoDoTurno/airflow,sid88in/incubator-airflow,subodhchhabra/airflow,MortalViews/incubator-airflow,danielvdende/incubator-airflow,airbnb/airflow,yk5/incubator-airflow,wileeam/airflow,OpringaoDoTurno/airflow,wooga/airflow,MortalViews/incubator-airflow,bolkedebruin/airflow,owlabs/incubator-airflow,DinoCow/airflow,fenglu-g/incubator-airflow,RealImpactAnalytics/airflow,janczak10/incubator-airflow,mtagle/airflow,zack3241/incubator-airflow,DinoCow/airflow,airbnb/airflow,spektom/incubator-airflow,jfantom/incubator-airflow,sid88in/incubator-airflow,jfantom/incubator-airflow,MortalViews/incubator-airflow,artwr/airflow,wooga/airflow,apache/incubator-airflow,mrares/incubator-airflow,jgao54/airflow,Fokko/incubator-airflow,mtagle/airflow,dhuang/incubator-airflow,spektom/incubator-airflow,wndhydrnt/airflow,cjqian/incubator-airflow,mrkm4ntr/incubator-airflow,owlabs/incubator-airflow,zack3241/incubator-airflow,apache/incubator-airflow,yati-sagade/incubator-airflow,malmiron/incubator-airflow,malmiron/incubator-airflow,spektom/incubator-airflow,nathanielvarona/airflow,malmiron/incubator-airflow,cjqian/incubator-airflow,Fokko/incubator-airflow,ProstoMaxim/incubator-airflow,subodhchhabra/airflow,Acehaidrey/incubator-airflow,wileeam/airflow,criccomini/airflow,KL-WLCR/incubator-airflow,adamhaney/airflow,airbnb/airflow,OpringaoDoTurno/airflow,skudriashev/incubator-airflow,andyxhadji/incubator-airflow,mrkm4ntr/incubator-airflow,wolfier/incubator-airflow,danielvdende/incubator-airflow,r39132/airflow,Tagar/incubator-airflow,adamhaney/airflow,lyft/incubator-airflow,adamhaney/airflow,jfantom/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,owlabs/incubator-airflow,andyxhadji/incubator-airflow,akosel/incubator-airflow,apache/airflow,mtagle/airflow,lxneng/incubator-airflow,hgrif/incubator-airflow,lxneng/incubator-airflow,cjqian/incubator-airflow,OpringaoDoTurno/airflow,mrkm4ntr/incubator-airflow,mistercrunch/airflow,zack3241/incubator-airflow,mistercrunch/airflow,bolkedebruin/airflow,lxneng/incubator-airflow,yati-sagade/incubator-airflow,cfei18/incubator-airflow,criccomini/airflow,CloverHealth/airflow,danielvdende/incubator-airflow,wolfier/incubator-airflow,gtoonstra/airflow,apache/airflow,danielvdende/incubator-airflow,yati-sagade/incubator-airflow,wndhydrnt/airflow,mrares/incubator-airflow,sid88in/incubator-airflow,MortalViews/incubator-airflow,Tagar/incubator-airflow,mrares/incubator-airflow,fenglu-g/incubator-airflow,CloverHealth/airflow,jhsenjaliya/incubator-airflow,janczak10/incubator-airflow,jgao54/airflow,gtoonstra/airflow,subodhchhabra/airflow,apache/airflow,gilt/incubator-airflow,wileeam/airflow,artwr/airflow,ProstoMaxim/incubator-airflow,criccomini/airflow,CloverHealth/airflow,bolkedebruin/airflow,Fokko/incubator-airflow,jgao54/airflow,RealImpactAnalytics/airflow,yati-sagade/incubator-airflow,malmiron/incubator-airflow,janczak10/incubator-airflow,dhuang/incubator-airflow,hgrif/incubator-airflow,Fokko/incubator-airflow,lyft/incubator-airflow,artwr/airflow,DinoCow/airflow,bolkedebruin/airflow,wileeam/airflow,Acehaidrey/incubator-airflow,wooga/airflow,andyxhadji/incubator-airflow,dhuang/incubator-airflow,sergiohgz/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,criccomini/airflow,mrares/incubator-airflow,apache/incubator-airflow,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